Necturus gallbladder epithelial cells activate a quiescent transport system in response to osmotically-induced cell shrinkage. This transport system is located on the apical surface of the cells when the osmotic change is made in the mucosal bath. We show that an osmotic challenge in the serosal bath also elicited transport across the apical membrane indicating that the site of transport is fixed even though the osmolality change occurs on the other surface of the cell.